Internal combustion engine comprise numberless elements that undergo friction and, as a result, undergo wear because they are subjected to severe stresses when the engine is functioning.
One of the ways to guarantee resistance to wear for an element that works sliding so that it can have long/sufficient useful life for the useful-life parameters of the engine is the application of one or more layers of coating on the base metal from which it is built. The coating, developed specifically for resisting wear and abrasion, maintains the performance properties of the element that works sliding, even after millions of explosion cycles of the engine.
Besides cylinders, pistons and rings, an internal combustion engine has a number of additional elements provided with at least one slide surface, which can receive coatings so as to prolong the useful life of the piece. Some of these elements are bronze bushings, cylinder bushings, components of the valve gear, etc.
In this regard, there are numberless techniques using the most varied compositions of coatings and numberless application processes, each trying to optimize the performance and durability properties of the most varied types and configurations of elements for use on an internal combustion engine having a slide surface.
In the spirit of the elements that work sliding and are object of the present invention, films/coatings of chrome nitride (CrN,Cr2N) used, for example, for piston rings applied to internal combustion engine, have typically “columnar” morphology (see FIG. 1) with the orthogonal direction to the substrate or metallic base that will receive the coating.
It should be noted that, since this is a coating applied to a surface that has relative sliding movement, for instance, reciprocating, a stress tangent to the free surface is intrinsic to the coating material because of the friction. In this condition, the stress of the coating due to friction takes place in the direction of less resistance of the coating because of its “columnar” texture.
Another condition that also affects the resistance of the films is the presence of pores (not shown in the figures). Pores are regions present in the coating, which have defect due to the extremely low localized cohesion of the material. Thus, the pores act as tension concentrators in greater or lesser scale, depending on the geometry and number of pores.
Thus, the orientation of the structure of the coating material influences its performance directly. Typically, cross-linking agents with (111) or (200) orientations are produced. The first orientation of the material (111) is known for exhibiting good resistance to wear, while the second orientation (200) contributes to minimizing internal tensions of the coating and, as a result, enables one to obtain a coating with more thickness than that obtained with coatings that nave only the orientation (111).
There are a few prior-art elements that work sliding and exhibit columnar morphology and orientation (111). Examples thereof are shown in documents U.S. Pat. No. 5,743,536 and U.S. Pat. No. 5,851,659, which describe piston rings provided with chrome-nitride coatings with columnar morphology and orientation (111) parallel to the coating surface. These documents disclose a coating having thickness that ranges from 1 to 80 micrometers (μm), with hardness ranging from 600 to 1000 Vickers (HV) and porosity ranging from 1.5 to 20%.
Additionally, there are also prior-art documents that present columnar morphology and both orientations (111) and (200). North-American document US 2009/278320A1 discloses a chrome-nitride coating with columnar morphology, provided with both orientations with a view to achieve benefits, mainly with regard to toughness. However, this prior-art document exploits a relatively small range of the (111)/(200) ratio, which ranges from 0.8 to 1.2. In other words, both orientations remain in an almost similar relationship, providing a film with thickness higher than 80 μm, hardness ranging from 1500 to 2500 HV and porosity higher than 10%.
Finally, document U.S. Pat. No. 6,372,369 describes an element that works sliding for use on an internal combustion engine (piston ring), which element is provided with a chrome-nitride coating (CrN) and titanium nitride (TiN) containing oxygen and that also exhibits a columnar morphology. This coating has a thickness ranging from 1 to 100 μm and hardness ranging from 1300 to 2300 HV. Such characteristics are achieved through an orientation of the coating with predominance of orientations (111) or (200). Thus, the chrome-nitride and titanium-nitride crystals have a preferred orientation (200) or (111) parallel to the surface to be coated, its morphology being thus columnar, from the base material to the outer surface of the coating.
In spite of the technological advances evidenced by the prior-art documents, one has not yet found a solution that could enable coatings of elements that work sliding applied to internal combustion engines with an increase of their resistance to wear, while preserving toughness and guaranteeing porosity lower than 1% or virtually zero.
Additionally, besides the fact that the prior art does not achieve coatings that provide extreme resistance to wear for sliding elements, enabling them to perform their functions adequately during long functioning cycles, such documents do not disclose the possibility of using coatings with alternative morphologies. Such morphologies are more capable of resisting the tribological conditions to which the sliding elements are subject, providing more durable conditions of functioning and enabling the internal combustion engines exhibit greater efficiency and lower emission of polluting gases.
Thus, the solutions of the prior art have two great drawbacks, which have not been eliminated so far. On the other hand, the increase in resistance to wear impairs the toughness of the coating and, on the other hand, the coatings exhibit porosity that is harmful to the durability of the coating. In this regard, the variables: resistance to wear, toughness and porosity are limited to an equation that needs the introduction of a new paradigm in order to achieve better results, namely at the level of the morphology of chrome-nitride coatings and the orientation of their structure.